The multi-carrier functionality for High-Speed Packet Access (HSPA) is evolving for each release of the 3GPP specifications, starting with the so-called Release 8 (Rel-8) specifications. For Release 9 (Rel-9), one of the introduced features was the combining of dual-carrier High-Speed Downlink Packet Access with Multiple-Input/Multiple-Output (MIMO) support (DC-HSDPA-MIMO), providing up to two MIMO-capable downlink carriers. Release 10 (Rel-10) HSPA evolved further by introducing support for up to four MIMO-capable carriers (4C-HSDPA). For Release 11 (Rel-11), 3GPP is standardizing support for up to eight MIMO-capable carriers (8C-HSDPA).
HSDPA systems use HARQ techniques to detect errors and facilitate retransmission of erroneously received data at the Medium Access Control (MAC) layer. This approach is quicker than relying on retransmissions at the Radio Link Control (RLC) layer. In HSDPA, HARQ operates at the transport block level, which means that errors are reported for individual transport blocks, and retransmission of an entire transport block is scheduled in response to a reported error. Only a single ACK/NACK bit is required for reporting the received status of each transport block.
To keep the delays associated with a retransmission low, the receiver (the user equipment, or UE, in the case of HSDPA) should report as quickly as possible whether a transport block was successfully received and decoded. HSDPA utilizes a stop-and-wait feedback process, to keep the signaling overhead low. With this approach, an ACK/NACK bit for each transport block is transmitted to the base station (the Node-B, in 3GPP terminology) at a pre-defined fixed time period (about five milliseconds) after the reception of the block. Retransmissions are scheduled in response to ACK/NACK bits that indicate a failed reception.
To allow for continuous data flow, HSDPA allows up to eight HARQ processes to run simultaneously. Each process is numbered and has its own buffer. The UE determines which HARQ process a given transport block belongs to from the downlink control signaling, and routes it to the appropriate buffer. With this approach, if a transport block is unsuccessfully received on one HARQ process, new data can continue to be sent on other HARQ processes even while decoding, ACK/NACK feedback signaling, and retransmission takes place for the first process.
The ACK/NACK feedback for HSDPA is transmitted by the UE on the High-Speed Dedicated Physical Control Channel (HS-DPCCH), which is an uplink channel specifically created to support HSDPA. This physical channel is transmitted on a separate channelization code using code-division multiplexing (CDM).
As specified in Rel-5 of the HSDPA specifications, the HS-DPCCH uses a spreading factor (SF) of 256, meaning that each data bit to be sent over the channel (a “channel bit”) is “spread” (i.e., multiplied) by a 256-bit spreading sequence, i.e., 256 “chips” are transmitted for each bit. Since the transmitted chip rate is 3.84 million chips per second (Mcps) and the HS-DPCCH is organized into two-millisecond sub-frames, 30 channel bits are sent in each HS-DPCCH sub-frame.
In the case of conventional single-carrier, single-input single-output (SISO) HSDPA, the ACK/NACK bit for a single transport block is encoded to ten bits, for increased reliability, and transmitted in the first third (the first slot) of the HS-DPCCH sub-frame. The codebook mapping an ACK codeword and a NACK codeword to ten encoded bits is simple in this case, as an ACK is represented by a sequence of ten 1's while a NACK is represented by a sequence of ten 0's. Channel Quality Information (CQI) is transmitted in the remaining twenty bits of the HS-DPCCH sub-frame.
Rel-7 of the HSDPA specifications introduced support for MIMO transmissions. Specifically, a dual-stream transmit adaptive array (D-TxAA) approach was defined, supporting simultaneous transmission of two independent data streams to capable terminals under appropriate signal conditions. With HSDPA-MIMO, up to two transport blocks can be simultaneously transmitted to a UE in any given transmission time interval (TTI).
HARQ processing is handled separately for each of the two simultaneously transmitted transport blocks in HSDPA-MIMO. This means that twice as much HARQ feedback is transmitted for the dual-stream transmission, since one HARQ acknowledgement per stream must be transmitted back to the Node-B. Thus, two ACK/NACK messages are jointly coded to form ten channel bits, and transmitted in the same slot used for the single-stream ACK/NACK message. This results in a slightly more complicated codebook, as four possible combinations of ACK/NACK messages are mapped to the ten available channel bits.
Support for multi-carrier transmission in HSDPA further complicates the HARQ feedback process. Rel-8 of the 3GPP standard introduced support for dual-carrier (or “dual-cell”) HSDPA transmissions. When dual-carrier support is coupled with MIMO techniques in Rel-9, up to two data streams can be transmitted on each carrier. This means that ACK/NACK feedback for as many as four transport blocks must be signaled to the base station, preferably using the same physical resources.
3GPP's solution to this was to encode all of this ACK/NACK feedback into the same ten channel bits used previously. The result is a significantly more complex codebook, including forty-eight codewords to account for all the possible combinations of ACK, NACK, and DTX (no transmission) states.
Support for four carrier (4C)-HSDPA transmission with MIMO was introduced in Rel-10 of the 3GPP standard. With up to two streams per carrier, a total of eight transport blocks could be received simultaneously in a sub-frame by a UE. As a result, the spreading factor was reduced by 3GPP from 256 to 128. This means that each bit is “spread” with a 128-bit long sequence. With the reduction in spreading factor, a total of 60 bits can be transmitted in a sub-frame. The HARQ-ACK information is transmitted in first third (the first slot) of the sub-frame, which carries a total of 20 bits.
To encode the HARQ-ACK message into a codeword transmitted on the HS-DPCCH, the activated carriers are divided into two groups. The first group consists of the HARQ-ACK information related to the primary serving high speed-downlink shared channel (HS-DSCH) carrier or “cell” and the 2nd secondary serving HS-DSCH carrier or “cell”. The second group consists of the HARQ-ACK information of the 1st and 3rd secondary serving HS-DSCH carriers or “cells”.
Each of these groups is encoded based on the Rel-9 codebook as illustrated in FIG. 1 which shows a structure of the SF128 HS-DPCCH subframe used in Rel-10. In FIG. 1, the primary serving HS-DSCH cell (or simply “serving cell”) is denoted c1. The 1st, 2nd and 3rd secondary serving cells are respectively denoted c2, c3 and c4. As seen, the HARQ-ACK information of the cells c1 and c3 (first group) are encoded jointly according to the Rel-9 codebook. Similarly the HARQ-ACK information of the cells c2 and c4 (second group) are also encoded jointly according to the Rel-9 codebook.
A “cell” in this context describes a combination of a signal carrier (or communications channel) and a geographical serving area of a base station, e.g., Node-B. Thus, a “cell” in this context is distinguished from a “sector” which is used to describe the serving area, i.e., multiple carriers covering the same area. An example site might include three sectors, each sector having N carriers, where N is the number of carriers deployed.
In order to avoid only half-slot transmission in the 1 slot carrying the HARQ-ACK information shown in FIG. 1, a new DTX codeword [0 0 1 1 0 1 1 0 1 0] was introduced in Rd-10. This DTX codeword is used when the UE does not detect any HS-DSCH transmissions for the cells constituting one of the first group c1/c3 or the second group c2/c4 and at the same time detects HS-DSCH transmissions for at least one cell belonging to the other of the first or the second group.
With the introduction of eight carrier (8C)-HSDPA, the problem of reliably encoding ACK/NACK feedback becomes even more challenging. Thus, a new acknowledgement and negative acknowledgement signaling solution is desirable to support the handling of retransmissions. To accommodate the additional downlink feedback, the following was agreed to at the 3GPP TSG-RAN WG1 meeting #65 (in Barcelona, Spain, 9th-13th May 2011). First, it was agreed to use an inphase (I)/quadrature phase (Q)-multiplexed SF128 HS-DPCCH solution in which HARQ-ACK information for up to four carriers is transmitted on the Q-branch of the used channelization code and the HARQ-ACK information for the remaining carriers is transmitted on the I-branch of the used channelization code. Second, it was agreed that the HARQ-ACK information transmitted on the I-branch and on the Q-branch are both encoded as done in Rel-10.
The agreements related to the HS-DPCCH HARQ-ACK coding for 8C-HSDPA are summarized in LS R1-111995 (R1-111995, “LS on the RAN1 agreements for 8C-HSDPA”, Barcelona, Spain, 9th-13th May, 2011), which is incorporated by reference in its entirety. Those agreements specify: “Working assumptions that the Rel-9/10 HARQ-ACK codebooks are reused for HARQ-ACK transmissions is confirmed.” That is, Rel-11 will reuse the existing Rel-9/10 HARQ-ACK codebooks.
An overview of the HS-DPCCH format used for Rel-11 is illustrated in FIGS. 2 and 3. FIG. 2 illustrates the spreading of HS-DPCCH when four or more secondary serving HS-DSCH cells are activated. FIG. 3 illustrates a structure of the HS-DPCCH sub-frame when four or more secondary serving HS-DSCH cells are activated. In this figure, the HARQ-ACK information related to the primary serving HS-DSCH cell and the 1st, 2nd, and 3rd secondary serving HS-DSCH cells (denoted c1, c2, c3 and c4, respectively) is inserted into the first subframe time slot of one of the branches (the Q-branch in FIG. 3) of the HS-DPCCH and the HARQ-ACK information related to the 4th, 5th, 6th, and 7th secondary serving cells (denoted c5, c6, c7 and c8, respectively) is inserted into the first subframe time slot of the other branch (the I-branch) of the HS-DPCCH.